Corrosion of metallic surfaces, such as down hole tubulars, during well operations is not uncommon and is evidenced by surface pitting, localized corrosion and loss of metal. Such metallic surfaces are typically composed of carbon steels, ferritic alloy steels, and high alloy steels including chrome steels, duplex steels, stainless steels, martensitic alloy steels, austenitic stainless steels, precipitation-hardened stainless steels and high nickel content steels.
Solid-free brines, commonly used in drilling and completion fluids, are typically high density brines. Brines typically used in completion and work-over fluids are tabulated in Table I with their respective density range:
TABLE IBrine DensityRange, pounds perAqueous Brine Compositiongallon (ppg)KHCO28.3-13.3NaHCO28.3-10.9NaBr8.3-12.7NaCl/NaBr10.0-12.7 CaCl28.3-11.6CaBr28.3-15.3CaCl2/CaBr211.6-15.1 CaCl2/CaBr2/ZnBr215.1-19.2 CaBr2/ZnBr214.2-19.2 CsHCO28.3-19.2
Such brines, especially higher density brines (like calcium chloride, calcium bromide, zinc bromide and mixtures thereof), have a high salt content and thus are highly corrosive. Marked corrosivity may be seen, for instance, when such brines are used as packer fluids since they remain in contact with production tubing and casing for an extended period of time.
The high corrosivity demonstrated by use of such high-density brines may cause a failure of down hole tubulars. Conventionally, a corrosion inhibitor or a corrosion inhibitor package is added to the brine to prevent or minimize brine corrosion on such metallic surfaces. Typically, the corrosion inhibitor or corrosion inhibitor package is added before or during the well operation.
Two types of inhibitors are conventionally used: film-forming amines and low molecular weight inorganic thiocyanate (SCN−) compounds. Film-forming amine inhibitors are often more effective when used at temperatures below 250° F. while the low-molecular weight inorganic thiocyanate inhibitors typically provide corrosion protection up to 350° F.
For the past twenty years, it is the sulfur containing corrosion inhibitors which have dominated the industry in light of their low cost and high efficiency. Unfortunately, sulfur-related stress corrosion cracking may occur from use of such corrosion inhibitors. Corrosion cracking translates into tubular failures. Even when no hydrogen sulfide is produced in the well, the thermal decomposition of sulfur-containing inhibitors may lead to sulfur-related stress corrosion cracking. Such inhibitors decompose at elevated bottomhole temperatures and release hydrogen sulfide. The release of hydrogen sulfide as a decomposition product is believed to induce sulfide stress corrosion cracking of the alloy tubulars. An increase in tubular failures due to stress corrosion cracking is further partially attributed to the use of high strength alloys such as 13 Chromium materials (e.g. modified 13 Cr and super 13 Cr tubulars with yield strengths of 95 and 110 ksi or more, respectively).
Efforts have been undertaken to find alternative corrosion inhibitors for use with high density brines which are capable of controlling, reducing or inhibiting corrosion without inducing sulfur-related corrosion cracking of metallic alloy tubulars. Such efforts have been principally focused on the development of sulfur-free corrosion inhibitors.